Plastic Electro-Optic Modulators Show Potential

Although the need for modulation at terahertz rates for telecommunications may be years away, researchers at Lucent Technologies' Bell Labs in Murray Hill, N.J., are working on plastic electro-optic modulators to do just that. Recently, Mark Lee and his colleagues tested a polymer modulator at rates of approximately 150 GHz. A practical device is still a distant dream, but the work demonstrates the potential use of polymers for high-speed electro-optics.

Electro-optic modulators encode data into an optical signal for transmission over fiber optic cable, so long-term increases in bandwidth usually point to higher-speed encoding and decoding.

Electro-optic materials themselves are similar. "When you apply an electric field to them, you change the speed of an optical lightwave propagating through the material," Lee explained. This change in speed usually is measured by a change in the material's index of refraction, and a larger change in the refractive index often means a better material.

He said that the most widely used modulator is made of lithium niobate, but as researchers search for better electro-optic materials, they have begun to examine polymers such as polymethylmethacrylate (PMMA), a relatively common plastic, as a host for chromophores. The Bell Labs modulator features a PMMA core bonded with the chromophore DR1, with a waveguide etched into it in a Mach-Zehnder interferometer geometry that is sandwiched between glass resin claddings.

Choice of polymer

The group carefully chose the polymer materials and the device construction to minimize degradation of the voltage signal at high frequencies. Doing this, Lee said, requires two things from an electro-optic dielectric material.

"One, you need the dielectric to absorb very little of the power in the drive voltage signal," he said. "Two, you need the drive voltage signal to propagate through the device at the same speed as the optical carrier wave."

The researchers tested the device for a 1310-nm carrier wavelength in a controlled laboratory setting and modeled the results with a Sonnet Software electromagnetic simulator. They modulated the device using standard sine wave applied to microwave synthesizers from speeds up to 40 GHz and specially constructed Gunn oscillators from 90 to 145 GHz. They also modulated it at 1.6 THz using a molecular gas far-infrared laser.

Although the resulting modulation at speeds of 150 GHz and detectable modulation at 1.6 THz are encouraging, Lee said that the device's signal-to-noise ratio is too low for practical use. Much more research remains to be done before polymer modulators become a reality in telecommunications systems, he said.

"Polymer photonics will remain in the research laboratory until someone produces a fully packaged device that passes Bellcore/Telcordia reliability specifications," he said. "This is far from a trivial issue and may involve obtaining a better understanding of how the polymers' [electro-optic] properties degrade with time, environment and use."

Polymer devices also must demonstrate that they can displace entrenched technologies on a cost or performance basis, he said. "I think we have identified performance advantages in terms of high-speed operation, but other possible advantages and disadvantages remain to be investigated."